This invention relates, in general, to bipolar transistors, and more particularly to bipolar transistors capable of operating in the microwave frequency range.
A typical bipolar transistor failure mode is degradation of the transistor current gain, commonly referred to as the transistor beta. The primary cause of beta degradation in a bipolar transistor is avalanche breakdown of the emitter-base junction due to operation of this junction under reverse bias conditions. Although operation under these conditions is not necessarily catastrophic to the transistor, the degradation in the value of beta may adversely affect small signal parameters such as transconductance and input resistance.
Most small signal parameters of a bipolar transistor amplifier are a function of the dc current in the emitters of these devices. Further, the dc emitter current in these devices is a function of beta. Therefore, beta degradation alters small signal parameters of bipolar transistors by changing the dc emitter current in the transistor. The degradation of beta will result in increased distortion, increased noise figure and ultimately in the reduction of the amplifier's gain.
Accordingly, it would be advantageous to have a beta degradation hardened bipolar transistor circuit capable of handling transients or intentional overvoltages appearing at the input. Beta degradation hardened transistors receiving excessive voltage levels at their inputs would be more reliable and require less frequent replacement. Finally, it would be advantageous that the beta degradation hardened transistors have device characteristics equal to or better than those of bipolar transistors currently available.